Ask your development team to make your website suitable to run on PHP 5. For documentation on migration from PHP4 to PHP5, please check migration guide available on PHP website.
This is one problem area which most of the small designers / design companies face one time or the other. First of all you, as a designer, have to analyze as why the client didn’t pay??
The project was delayed?? Finished Project was not as discussed earlier?? If yes, then what caused this?